<PAGE>   1
          Confidential materials omitted and filed separately with the
                       Securities and Exchange Commission.
                        Asterisks denote such omissions.

                        DATED this 16th day of March 1997

                                     BETWEEN

                               BIOTRANSPLANT INC.
                                     ("BTI")

                                      -and-

                            CASTELLA RESEARCH PTY LTD
                                  ("Castella")

                                     - and-

                             SECURE SCIENCES PTY LTD
                                   ("Secure")

                                     - and-

                           STEM CELL SCIENCES PTY LTD
                                 ("the Company")




                AGREEMENT TO FURTHER VARY SHAREHOLDERS' AGREEMENT



                                 HOLDING REDLICH
                             LAWYERS AND CONSULTANTS

         350 Williams Street                         Level 12, Chifley Tower
         Melbourne VIC 3000                          2 Chifley Squre
                                                     Sydney NSW 2000

         Tel (03) 9321 9999                          Tel (02) 234 4444
         Fax (03) 9321 9900                          Fax (02) 234 4400

         Ref:  CLSCS113.27

<PAGE>   2
AGREEMENT TO FURTHER VARY SHAREHOLDERS' AGREEMENT dated the
     day of                , 1997

BETWEEN:

              BIOTRANSPLANT INCORPORATED a corporation organised and existing
              under the laws of the State of Delaware and having its principal
              office 13 Pipp Street, Building 96, Navy Yard, Charlestown, MA,
              United States of America 
                                                                         ("BTI")


AND:          CASTELLA RESEARCH PTY LTD (ACN 006 125 364) of 587 Whitehorse
              Road, Mont Albert, Victoria, Australia in its own capacity and as
              trustee of the M.R. Brandon Family Trust 
                                                                    ("CASTELLA")


AND:          SECURE SCIENCES PTY LTD (ACN 064 139 948) of Level 10, 420 St.
              Kilda Road, Melbourne, Victoria, Australia in its own capacity and
              as trustee of the Secure Sciences Unit Trust 
                                                                      ("SECURE")


AND:          STEM CELL SCIENCES PTY LTD (ACN 063 293 130) of Level 10, 420 St.
              Kilda Road, Melbourne, Victoria, Australia 
                                                                 ("THE COMPANY")

RECITALS



A.            By a Shareholders' Agreement dated 5 April, 1994 between BTI,
              Castella, Secure and the Company (THE "SHAREHOLDERS' AGREEMENT")
              the Shareholders recorded their agreement as to how the Company
              would be owned, controlled and funded by them.

B.            The Shareholders' Agreement was varied by Variations of the
              Shareholders' Agreement made on 2 February 1996 and 20 December
              1996 (the "Second Variation Agreement").


                                       2
<PAGE>   3
          Confidential materials omitted and filed separately with the
                       Securities and Exchange Commission.
                        Asterisks denote such omissions.

C.            Pursuant to the Second Variation Agreement:

              (a)           BIT was issued with an option to subscribe for
                            ****** in the capital of the Company for a total
                            consideration of US ****** at any time on or before
                            31 December, 1996;

              (b)           Castella was issued with an option to subscribe for
                            ****** in the capital of the Company for a total
                            consideration of ******at any time on or before 31
                            December, 1996; and

              (c)           Secure was issued with an opton to subscribe for
                            ******in the capital of the Compay for a total
                            consideration of ****** at any time on or before 31
                            December, 1996

D.            None of BTI, Castella and Secure have exercised the options issued
              to them pursuant to the Second Variation Agreement.

E.            BTI has agreed to provide an amount of equity capital to the
              Company.

F.            In line with the intent of the Shareholders' Agreement that on the
              provision by BTI of equity capital to the Company such shares in
              the Company would be issued to maintain the shareholdings of BTI,
              Castella and Secure in the Company in the ratios set out in
              Recital J, the Shareholders have agreed to cause the Company to
              allot to BTI a certain addition number of "A" shares in the
              capital of the Company at a premium and to allot to Castella and
              Secure certain additional "B" Shares in the capital of the Company
              at par.

G.            The Shareholders and the Company have also agreed that the Company
              will allot to BTI, Castella and Secure fresh options on the terms
              set out in this Agreement.

H.            Clause 18.5 of the Shareholders' Agreement provides that the
              Shareholders' Agreement may not be modified, amended, added to or
              otherwise varied except by a document in writing signed by each of
              the parties or signed on behalf of each party by a director under
              hand.

I.            The parties wish by this Agreement to record the matters set out
              in Recitals E to G and to vary the Shareholders' Agreement to the
              extent necessary to take account of those matters.


                                       3
<PAGE>   4
          Confidential materials omitted and filed separately with the
                       Securities and Exchange Commission.
                        Asterisks denote such omissions.

IT IS AGREED

1.            DEFINITIONS AND INTERPRETATION

1.1           Definitions

              In this Agreement (including in the Recitals), unless the contrary
              intention appears, each defined word and expression has the
              meaning assigned to that word or expression in the Shareholders'
              Agreement.

1.2           Interpretation

              Clauses 1.3 and 1.4 of the Shareholders' Agreement are
              incorporated in and form part of this Agreement as if each
              referenced to "this Agreement" in those clauses were a reference
              to this Agreement to vary the Shareholders' Agreements.

1.3           Recitals

              The parties acknowledge and agree that the Recitals are true and
              correct and accurately reflect the circumstances in which this
              Agreement was entered into.

2.            ISSUE OF SHARES

2.1           Issue of Shares to BTI

              On the date of this Agreement BTI must subscribe for, and be
              issued with **** ****** in the capital of the Company issued at a
              price of ****** per share (being $1.00 par and ****** premium per
              share) for a total consideration of ******

2.2           Issue of Shares to Castella

              On the date of this Agreement Castella must subscribe for and be
              issued with ****** in the capital of the Company issued at the par
              price of $1.00 per share for a total consideration of ******
              payable on the date of this Agreement.

2.3           Issue of Shares to Secure

              On the date of this Agreement Secure must subscribe for and be
              issued with ****** in the capital of the Company issue at the par
              price of $1.00 per share for a total consideration of ******
              payable on the date of this Agreement.


                                       4
<PAGE>   5
          Confidential materials omitted and filed separately with the
                       Securities and Exchange Commission.
                        Asterisks denote such omissions.

3.            ISSUE OF OPTIONS

3.1           Issue of Options to BTI

              (a)  On the date of this Agreement, the Company must issue to BTI
                   an option (the Option Terms attaching) to subscribe for,
                   fully pay up and be issued with a further ****** in the
                   capital of the Company for a total consideration of US******
                   at any time on or before 31 December, 1997 ("THE OPTION").

              (b)  If BTI exercises the option the Company will issue BTI ******
                   in the capital of the Company at a price per share of A$1.00
                   (being the par value) plus a premium per share calculated in
                   A$ as at the date of the exercise of the option.

3.2           Issue of Option to Castella

              On the date of this Agreement, the Company must issue to Castella
              an option (with the Option Terms attaching) to subscribe for,
              fully pay up and be issued with a further ****** in the capital of
              the Company issued at a price of $1.00 per share for a total
              consideration of ****** at any time on or before 31 December,
              1997.

3.3           Issue of Option to Secure

              On the date of this Agreement, the Company must issue to Secure an
              option (the Options Terms attaching) to subscribe for, fully pay
              up and be issued with a further ****** in the capital of the
              Company issued at a price of $1.00 per share for a total
              consideration of ****** at any time on or before 31 December,
              1997.

4.            VARIATION OF SHAREHOLDERS' AGREEMENT

4.1           Variation

              In consideration of the mutual agreements made by each party under
              this Agreement, the parties agree, pursuant to cause 18.5 of the
              Shareholders' Agreement, to vary clause 2.5 of the Shareholders'
              Agreement to the extent necessary to give effect to the provisions
              of Clauses 2 and 3 of this Agreement.


                                       5
<PAGE>   6
4.2           Confirmation of Shareholders' Agreement

              The parties confirm that the terms and conditions of the
              Shareholders' Agreement (as varied by this Agreement) remain in
              full force and effect.

SIGNED AS AN AGREEMENT on the date first appearing

SIGNED by BIOTRANSPLANT INC.            )

by its director ELLIOT LEBOWITZ         )     /s/ Elliot Lebowitz
                                         ------------------------------
in the presence of:                     )


(Witness)


SIGNED by CASTELLA RESEARCH             )

PTY LTD by its director MAL BRANDON     )       /s/ Mal Brandon
                                         ------------------------------
in the presence of:                     )


(Witness)


SIGNED by SECURE SCIENCES PTY LTD       )

by its director PETER MOUNTFORD         )     /s/ Peter Mountford
                                         ------------------------------
in the presence of:                     )


(Witness)


SIGNED by STEM CELL SCIENCES            )

PTY LTD by its director MAL BRANDON     )       /s/ Mal Brandon
                                         ------------------------------
in the presence of:                     )


(Witness)



                                       6

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